A polymer electrolyte fuel cell has a low working temperature of 70 to 100° C., a short operating time, and a high output density, thereby getting the spotlight as a power source for transport, a portable power source, a home power source, and the like, and a fuel cell stack includes a divider sheet including a membrane-electrode assembly including an electrolyte and electrodes (anode and cathode), and an end plate including an inlet/outlet of air and an inlet/outlet of hydrogen gas.
The fuel cell divider sheet is generally formed of one of graphite, a carbon complex, a Ti alloy, stainless steel, and conductive plastic. The stainless steel is also one of main materials of the fuel cell divider sheet. The stainless steel may have low interface contact resistance, superior corrosion resistance and thermal conductivity, and low gas transmissibility, be formed to have a large area, have superior product moldability, and be formed to be thin, thereby decreasing volume and weight of the fuel cell stack.
The metal divider sheet using stainless steel is subjected to a process of forming a channel provided with a flow path by using a material generally having a small thickness of around 0.1 mm by using stamping and hydroforming processes, unlike to a process of designing and manufacturing a flow path of a graphite divider sheet by using a mechanical machining method.
In the thin plate stainless steel, which is subjected to the aforementioned molding process, moldability of a material needs to be superior, there have to be no surface defect in a product after molding, and a molding deformed portion needs not to have necking and fracture even under a design requirement of various molding flow path depths and channel widths.
In terms of the moldability of the stainless steel thin plate product, there is a fracture phenomenon by local concentration of stress of the material by stretcher strain and the like by yield point elongation of the material depending on a section of plastic deformation applied to the material, and a moldability problem due to a surface defect or elongation deterioration by a non-uniform deformation pattern of a surface. The stretcher strain defect generated by yield point elongation of metal among the factors is a phenomenon in which non-uniform deformation of the material is incurred by the small amount of interstitial solid solution elements of the material, an intaglio pattern shaped like a flame is represented on a surface, and thus the entire surface becomes rough while the deformation continues, and this phenomenon may cause a defect by fine wrinkles formed at the channel portion, in which the flow path of the divider sheet is molded, or generation of fracture by local concentration of stress to a deformed portion of the material deformed area during the molding of the fuel cell divider sheet, so that a fundamental solution is demanded.
Accordingly, removal of yield point elongation may be considered as an essential element for improving moldability during the molding of the fuel cell divider sheet. In general, in order to remove yield point elongation, a method of removing yield point elongation by cold rolling or leveling a final rolled sheet material by 0.5 to 2%. However, there is a problem in that manufacturing cost of a material may be increased due to an additional process, such as cold rolling or leveling, and yield point elongation may be re-generated after a predetermined time.
An object of the present invention is to provide stainless steel for a fuel cell divider sheet having superior moldability, which has no stretcher strain by yield point elongation of a material, has superior elongation, and has no fracture by local concentration of stress to a deformed region of the material during molding of a flow path of the divider sheet for a thin plate material.
Another object of the present invention is to provide a method of manufacturing stainless steel, which has superior surface quality, in addition to moldability, to be used for a divider sheet of a fuel cell for a vehicle, home, and a portable use.